Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys* 1

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1 Materials Transactions, Vol. 5, No. 12 (9) pp to 273 Special Issue on Low Cost Reduction Processes, Roles of Low Cost Elements and Interstitial Elements, and Microstructural Control for Generalization of Titanium Alloys #9 The Japan Institute of Metals Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys* 1 Masaki Tahara 1; * 2, Hee Young Kim 1; * 3, Tomonari Inamura 2, Hideki Hosoda 2 and Shuichi Miyazaki 1;3; * 3 1 Institute of Materials Science, University of Tsukuba, Tsukuba , Japan 2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama , Japan 3 School of Materials Science and Engineering, Gyeongsang National University, 9 Gazwadong, Jinju, Gyeongnam 66-71, Korea Recently, the Ti-Zr-Nb alloys have been developed as Ni-free shape memory and superelastic alloys. In this study, the effects of Nb and nitrogen (N) contents on martensitic transformation behavior, shape memory effect and superelasticity in Ti-18Zr-(1216)Nb-(1:)N (at%) alloys are investigated using loading and unloading tensile tests, optical microscopy and X-ray diffractometry. The shape memory effect is observed in Ti-18Zr-(1213)Nb and Ti-18Zr-12Nb-.5N alloys at room temperature. The superelastic behavior appears by the increase of Nb or N content. The Ti-18Zr-(1415)Nb, Ti-18Zr-(1314)Nb-.5N and Ti-18Zr-(1214)Nb-1.N alloys exhibit the superelasticity at room temperature. The martensitic transformation start temperature decreases by 75 K with 1 at% increase of N content for the Ti-18Zr-13Nb alloy. The critical stress for slip deformation and the stress for inducing the martensitic transformation increase with increasing N content. The superelastic recovery strain is also increased by adding N. The maximum recovery strain of 5.% is obtained in the alloy. [doi:1.232/matertrans.ma97] (Received June 3, 9; Accepted August 5, 9; Published November 25, 9) Keywords: shape memory alloys, superelasticity, biomaterials, smart material, titanium based alloy, titanium-zirconium-niobium-nitrogen, interstitial element 1. Introduction Superelasticity has been usefully applied in medical devices such as orthodontic arch wires, stents and guide wires. Among many shape memory alloys, only Ti-Ni alloys have been successfully applied in the biomedical field owing to a large shape recovery strain and high corrosion resistance. However, the risk of Ni-hypersensitivity after implantation has been pointed out because Ti-Ni alloys contain about 5 at% of Ni. This has led to the development of Ni-free -Ti base shape memory alloys which consist of only non-toxic elements. 1 12) In -Ti alloys, superelasticity is associated with a stress induced martensitic transformation from the phase (bcc) to martensite phase (orthorhombic) by loading and its reverse transformation by unloading. The control of the martensitic transformation temperatures by the adjustment of the amount of -stabilizer elements such as Nb, Mo and Ta is necessary to obtain the superelasticity in Ti-base alloys. In Ti-Nb binary alloys, superelastic behavior was observed at room temperature when the Nb content is 2627 at% Nb. 1) The superelasticity has been also reported in ternary alloys such as Ti-Nb-Sn, 2) Ti-Nb-Zr, 3) Ti-Nb-Ta, 4) Ti-Nb-Al, 5) Ti-Nb-Pt 6) and Ti-Nb-O. 7) In addition, it has been also reported that the superelasticity was observed in Ti-Mobased alloys such as Ti-Mo-Sn, 8) Ti-Mo-Sc 9) and Ti-Mo- Ga. 1) However, these -Ti base superelastic alloys exhibit the shape recovery strain of only 3% at the maximum. We have reported that the addition of Zr to Ti-Nb base alloys as a substitute of Nb is effective to increase the transformation strain with keeping the martensitic transformation start * 1 This Paper was Originally Published in Japanese in J. Japan Inst. Metals 72 (8) * 2 Graduate Student, University of Tsukuba * 3 Corresponding authors: heeykim@ims.tsukuba.ac.jp, miyazaki@ims. tsukuba.ac.jp temperature (Ms) similar. 11,12) However, the shape recovery strain of the Zr added alloys was still small (3:5%) because the critical stress for slip of these alloys was low. Therefore, it is necessary to increase the critical stress for slip in order to obtain a large shape recovery strain. On the other hand, we have reported that the addition of interstitial elements such as oxygen (O) and nitrogen (N) to Ti-Nb base alloys is effective to increase the shape recovery strain due to the increase of critical stress for slip. 7,13) Furuhara et al. also reported that the addition of N to the Ti-1V-2Fe-3Al alloy (mass%) increases the yield stress significantly. 14) In this study, N was added to Ti-Zr-Nb alloys in order to increase the critical stress for slip and improve the superelasticity. The effects of N addition on the shape memory effect, superelastic behavior, phase stability and mechanical properties were investigated by tensile tests, optical microscopy and X-ray diffractometry. 2. Experimental Procedures Ti-18Zr-(1216)Nb, Ti-18Zr-(1215)Nb-.5N and Ti- 18Zr-(1214)Nb-1.N alloys (at%) were prepared by the Ar arc melting method. The N content was controlled by the amount of TiN. The changes in weight due to arc melting were less than.2 mass%, thus, it is considered that N content did not change significantly by arc melting in this study. It was also reported that there was no significant change in the N content by arc melting in Ti-1V-2Fe-3Al-N alloys (mass%). 14) The ingots were sealed in a vacuum quartz tube and homogenized at 1273 K for 7.2 ks, and cold-rolled with a reduction of 98.5% in thickness. The thickness of coldrolled sheets was about 15 mm. Specimens for tensile tests, optical microscope (OM) observation and X-ray diffraction (XRD) measurements were cut using an electro-discharge machine. The damaged surface was removed by chemical

2 Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys 2727 (N content) 1. N.5 N 12Nb 13Nb 14Nb 15Nb 16Nb (Nb content) Apparent Yield N.5N N N etching. The specimens for Ti-18Zr-(1216)Nb and Ti- 18Zr-(1215)Nb-.5N alloys were solution treated at 1173 K for 1.8 ks in an Ar atmosphere, followed by quenching into water. On the other hand, the specimens for Ti-18Zr- (1214)Nb-1.N alloys were solution treated at 1273 K for 1.8 ks in order to avoid the precipitation of phase. Tensile tests were carried out at a strain rate of 2:5 1 4 s 1 at temperatures between 195 and 295 K. After loading-unloading tensile tests, the specimens which did not exhibit superelastic recovery were heated up to about 5 K. The gage length of the specimens was 2 mm. XRD measurements were conducted at room temperature with Cu K radiation. The surface of the specimens for OM observation was electro-polished using a solution of perchloric acid : butanol : methanol ¼ 1: 6: 1 in volume at 233 K, followed by chemical etching in a solution of fluorinated acid : nitric acid : water ¼ 1:1:8in volume at room temperature. 3. Results and Discussion Fig. 1 Stress-strain curves obtained at room temperature for Ti-18Zr- (1216)Nb (at%), Ti-18Zr-(1215)Nb-.5N (at%) and Ti-18Zr- (1214)Nb-1.N (at%) alloys. 3.1 Effect of composition on shape memory effect and superelastic behavior The shape memory effect and superelastic behavior of Ti- Zr-Nb-N alloys were investigated. Figure 1 shows the stressstrain curves of Ti-18Zr-(1216)Nb, Ti-18Zr-(1215)Nb-.5N and Ti-18Zr-(1214)Nb-1.N alloys obtained by loading and unloading tensile tests at room temperature. The tensile stress was applied until the strain reached about 2.5%, and then stress was removed. After unloading, the specimens which did not recover superelastically were heated up to about 5 K: dashed lines with an arrow indicate the shape recovery by heating. In N alloys, the shape memory effect was observed in the 12Nb and 13Nb alloys. The superelasticity was obtained in the 14Nb and 15Nb alloys. This means that the addition of Nb decreased the transformation temperatures. The similar tendency was Nb content (at%) Fig. 2 Nb content dependence of apparent yield stress for Ti-18Zr- (1216)Nb, Ti-18Zr-(1215)Nb-.5N and Ti-18Zr-(1214)Nb-1.N alloys. observed in the.5n and 1.N alloys. The effect of Nb content in Ti-Nb binary alloys on the transformation temperature has been reported: 1 at% Nb addition decreases Ms by4k. 1) The addition of.5 or 1. at% N caused the Ti-18Zr-(12, 13)Nb alloys to exhibit the superelasticity. This means that the addition of N was also effective to decrease the transformation temperatures. The Ti-18Zr-16Nb and -.5N alloys did not exhibit superelasticity because the Ms of these alloys were too low and plastic deformation occurred prior to the stress induced martensitic transformation. In addition, these two alloys exhibited serrated yielding. It is considered that the serration is related to the twining deformation, however, the further investigation is required. The apparent yield stress (.2% proof stress) of each curve was plotted as a function of Nb content in Fig. 2. For the N alloys, the apparent yield stress decreased with increasing Nb content, and took a minimum at 13Nb. The apparent yielding of the Ti-18Zr-12Nb alloy corresponds to the reorientation of martensite variants because this alloy consisted mainly of the phase. On the other hand, the Ti-18Zr-(1315)Nb alloys consisted of the phase, indicating that the apparent yielding of the Ti- 18Zr-(1315)Nb alloys corresponds to the stress induced martensitic transformation. In particular, the Ms of the Ti- 18Zr-13Nb alloy was close to the test temperature (room temperature), thus the stress induced martensitic transformation easily occurred at low stress level because the phase of the Ti-18Zr-13Nb alloy was most unstable. The stress for inducing the martensitic transformation increased with increasing Nb content in the Ti-18Zr-(1315)Nb alloys, which is due to the fact that the Ms decreased with increasing Nb content. In general, the parent phase ( phase) becomes more stable with increasing test temperature when the test temperature is above Ms, thus a higher stress is required for inducing the martensitic transformation at a higher test temperature, which is consistent with the Clausius-Clapeyron relationship. On the other hand, when the test temperature is kept at room temperature, the stress for inducing the martensitic transformation increases with

M. Tahara, H. Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki (211)β Intensity (a.u.) ()β (11)β 2728 1.N 3 4 5 2θ 6 (22)α" ()α" (22)α" (111)α" (11)α".5N 7 N 8 Fig.

N alloys, the apparent yield stress increased with increasing Nb content without showing the negative Nb content dependence. It is considered that the composition showing the minimum point of.

The reﬂection peaks from both and phases were detected in the Ti-18Zr-12Nb alloy. On the other hand, only the phase was conﬁrmed in the Ti-18Zr- Fig. 4 12Nb-(.5, 1.)N alloys.

3 M. Tahara, H. Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki (211)β Intensity (a.u.) ()β (11)β N θ 6 (22)α" ()α" (22)α" (111)α" (11)α".5N 7 N 8 Fig. 3 XRD proﬁles obtained at room temperature in Ti-18Zr-12Nb( 1:)N alloys. decreasing Ms of the specimen. In case of the.5n and 1.N alloys, the apparent yield stress increased with increasing Nb content without showing the negative Nb content dependence. It is considered that the composition showing the minimum point of.5n and 1.N alloys was shifted to a lower Nb content because Ms decreased by the addition of N. The XRD proﬁles of Ti-18Zr-12Nb-( 1:)N alloys are shown in Fig. 3. The reﬂection peaks from both and phases were detected in the Ti-18Zr-12Nb alloy. On the other hand, only the phase was conﬁrmed in the Ti-18Zr- Fig. 4 12Nb-(.5, 1.)N alloys. It is also noted that the peaks from nitride-precipitates such as TiN were not detected in the Nadded alloys, thus it is considered that N was fully dissolved into the matrix. Figure 4 shows the OM images of (a) Ti18Zr-12Nb, (b) Ti-18Zr-12Nb-.5N and (c) Ti-18Zr-12Nb1.N alloys. A typical morphology of martensite was observed in the Ti-18Zr-12Nb alloy, while only the phase was observed in N-added Ti-18Zr-12Nb-(.5, 1.)N alloys, conﬁrming that the addition of N decreases the Ms. This is consistent with the results obtained by tensile tests (Fig. 1) and XRD measurements (Fig. 3). 3.2 Eﬀect of N addition on Ms temperature In order to evaluate the eﬀect of N addition on the Ms in detail, tensile tests were carried out at various temperatures for the Ti-18Zr-13Nb-( 1:)N alloys. Figure 5 shows the series of stress-strain curves obtained in the Ti-18Zr-13Nb.5N alloy. All the specimens exhibited the superelasticity and shape memory eﬀect. The apparent yield stress for each curve was plotted as a function of test temperature in Fig. 6. With increasing test temperature, the yield stress decreased to the minimum at 221 K, then increased with further increasing test temperature. In this study, the temperature exhibiting the minimum yield stress is deﬁned as Ms. Thus, the Ms of the Ti-18Zr-13Nb-.5N alloy was determined to be 221 K. The Ms temperatures of the Ti-18Zr-13Nb(, 1.)N alloys were determined by the same method and they were plotted against the N content in Fig. 7. Figure 7 reveals that the Ms decreased by 75 K with 1 at% increase of N content. It has been reported that the addition of 1 at% Nb decreases the Ms by 4 K. This means that the eﬀect of.5 at% N addition for decreasing the Ms is equivalent to that of 1 at% Nb addition. Optical micrographs of (a) Ti-18Zr-12Nb, (b) Ti-18Zr-12Nb-.5N and (c) Ti-18Zr-12Nb-1.N alloys.

4 Effect of Nitrogen Addition on Superelasticity of Ti-Zr-Nb Alloys 2729 Ti-18Zr-13Nb-.5N 295 K 266 K 249 K 232 K Apparent Yield K 221 K 213 K 195 K Fig. 5 Stress-strain curves obtained at various temperatures for a Ti-18Zr- 13Nb-.5N alloy. 16 Ms temperature Temperature, T / K Fig. 6 Temperature dependence of the apparent yield stress for a Ti-18Zr- 13Nb-.5N alloy. Martensitic Transformation Start Temperature, Ms / K K/1at.% N N content (at%) Fig. 7 Nitrogen content dependence of martensitic transformation start temperature (Ms) estimated by stress-strain curves for Ti-18Zr-13Nb- (1:)N alloys. 3.3 Superelastic properties of Ti-Zr-Nb-N alloys For the ternary,.5n added and 1.N added alloys, the best superelasticity was observed in the, Ti-18Zr- 14Nb-.5N and alloys, respectively, as σ SIM σ CSS Fig. 8 Stress-strain curves obtained at room temperature for Ti-18Zr- 15Nb, and alloys. shown in Fig. 1. It is suggested that these alloys have almost same Ms, because the decrease in Ms due to the addition of.5 at% N is compensated by the decrease of 1 at% Nb content. Thus, these three alloys were further investigated in order to clarify the effect of N addition on the superelastic properties. Figure 8 shows the stress-stain curves obtained at room temperature for the, and alloys. The alloy exhibited a large fracture strain of 5%. The fracture strain deceased with increasing N content and only of fracture strain was observed in the alloy, while the tensile strength increased with increasing N content because of the solid solution hardening effect of N. It is also seen that all the alloys exhibited a two-stage yielding. The stress for the first yielding indicated by an open arrow corresponds to the critical stress for inducing the martensitic transformation ( SIM ). On the other hand, the stress for the second yielding indicated by a solid arrow corresponds to the critical stress for slip ( CSS ). Both SIM and CSS increased with increasing N content. These results are consistent with the published reports on the effect of interstitial atoms on the superelastic properties of -Ti alloys. 7) In order to evaluate the superelastic behavior more quantitatively, the strain increment cyclic tensile tests were carried out at room temperature, and the results are shown in Fig. 9. At the first cycle, tensile stress was applied until the strain reached about 1.5%, and then the stress was removed. The similar measurement was repeated by increasing the maximum applied strain by.5% upon loading for the same specimen. The starting points of the stress-strain curves are shifted in order to separate each curve. Four types of strains were defined as follows: (1) the total shape recovery strain " r, (2) the remained strain after unloading " p, (3) the superelastically recovered strain due to the reverse transformation " se, and (4) the elastic strain recovered by elastically upon unloading " el.the" p drastically increased from the 5th cycle, 8th cycle and 7th cycle for the Ti-18Zr- 15Nb, and alloys, respectively. The magnitudes of " r and " se of each cycle were plotted as a function of the maximum tensile stress at each cycle in Fig. 1. The " r and " se increased with increasing tensile

5 273 M. Tahara, H. Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki ε p ε se ε el ε r Fig. 9 Stress-strain curves obtained by cyclic loading-unloading tensile tests for, and alloys Fig. 1 The recovery strain (" r ) and superelastic strain (" se ) as a function of the maximum tensile stress of each cyclic deformation. stress. For the alloy, the maximum " r (" r max )of 3.5% was obtained. On the other hand, the N-added alloys exhibited a larger " r max : 5.% for the alloy and 4.5% for the alloy. This is due to the increase of CSS by the addition of N. It is noted that the alloy has a higher CSS when compared with the alloy, however, " r max of the Ti- 18Zr-13Nb-1.N alloy is slightly smaller than that of the Ti- 18Zr-14Nb-.5N alloy. This can be explained by the fact that the increase of N raises not only CSS but also SIM : a higher SIM of the alloy led the alloy to deform plastically more easily when compared with the Ti-18Zr- 14Nb-.5N alloy. Based on the above results, it can be concluded that the N addition to Ti-Zr-Nb alloys was effective to improve the superelastic behavior due to the increase of the CSS and the addition of.5 at% N was more effective in the sense of a large recovery strain. ε r ε se 4. Conclusions The effects of Nb and N contents on martensitic transformation behavior, shape memory effect and superelasticity in Ti-18Zr-(1216)Nb-(1:)N alloys were investigated using loading-unloading tensile tests, optical microscopy and X-ray diffractometry. The following results were obtained. (1) The shape memory effect is obtained in the Ti-18Zr- (1213)Nb alloys at room temperature, and the increase of Nb content causes to exhibit the superelasticity, then the best superelasticity is observed in the alloy. The best superelasticity is observed in the and Ti-18Zr-13Nb- 1.N alloys among the.5n-added and 1.N-added alloys, respectively. (2) The addition of 1 at% N to the Ti-18Zr-13Nb alloy decreases the martensitic transformation start temperature by 75 K. (3) The critical stress for slip deformation and the stress for inducing the martensitic transformation increase with increasing N content. The superelastic recovery strain is also increased by adding N. The maximum recovery strain of 5.% is obtained in the Ti-18Zr- 14Nb-.5N alloy. Acknowledgements This work was partially supported by the Grants-in-Aid for Fundamental Science Research (Wakate B(6-7), Kiban C(8-21)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was also partially supported by World Class University Project from Korean Science and Engineering Foundation. REFERENCES 1) H. Y. Kim, Y. Ikehara, J. I. Kim, H. Hosoda and S. Miyazaki: Acta Mater. 54 (6) ) E. Takahashi, T. Sakurai, S. Watanabe, N. Masahashi and S. Hanada: Mater. Trans. 43 (2) ) J. I. Kim, H. Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki: Mater. Sci. Eng. A 43 (5) ) H. Y. Kim, S. Hashimoto, J. I. Kim, T. Inamura, H. Hosoda and S. Miyazaki: Mater. Sci. Eng. A 417 (6) ) Y. Fukui, T. Inamura, H. Hosoda, K. Wakashima and S. Miyazaki: Mater. Trans. 45 (4) ) H. Y. Kim, N. Oshika, J. I. Kim, T. Inamura, H. Hosoda and S. Miyazaki: Mater. Trans. 48 (7) ) J. I. Kim, H. Y. Kim, H. Hosoda and S. Miyazaki: Mater. Trans. 46 (5) ) T. Maeshima and M. Nishida: Mater. Trans. 45 (4) ) T. Maeshima and M. Nishida: Mater. Trans. 45 (4) ) H. Y. Kim, Y. Ohmatsu, J. I. Kim, H. Hosoda and S. Miyazaki: Mater. Trans. 45 (4) ) S. Miyazaki and H. Y. Kim: Mat. Sci. Forum (7) ) S. Miyazaki, H. Y. Kim and H. Hosoda: Mater. Sci. Eng. A (6) ) M. Tahara, H. Y. Kim, T. Inamura, H. Hosoda and S. Miyazaki: Proc. 11th World Conf. on Titanium, (JIMIC 5, 7) pp ) T. Furuhara, S. Annaka, Y. Tomio and T. Maki: Mater. Sci. Eng. A (6)

Martensitic Transformation and Superelasticity of Ti-Nb-Pt Alloys

Materials Transactions, Vol. 48, No. 3 (27) pp. 4 to 46 Special Issue on Smart and Harmonic Biomaterials #27 The Japan Institute of Metals Martensitic Transformation and Superelasticity of Ti-Nb-Pt Alloys